1. Field of the Invention
The present invention relates to a magnetic recording medium and a method for evaluating characterization of a thermally-assisted magnetic recording device that is provided with a thermally-assisted magnetic recording head.
2. Description of the Related Art
In the field of magnetic recording using a head and a medium, further performance improvements of thin film magnetic heads and magnetic recording media have been demanded in conjunction with a growth of high recording density of magnetic disk devices. Currently, composite type thin film magnetic heads are widely used for the thin film magnetic heads. The composite type thin film magnetic heads are configured with a configuration in which a magnetoresistive (MR) element for reading and an electromagnetic conversion element for writing are laminated.
The magnetic recording medium is a discontinuous medium in which magnetic grains are aggregated and each of the magnetic grains has a single magnetic domain structure. In this magnetic recording medium, a single recording bit is configured with a plurality of magnetic grains. Therefore, in order to increase recording density, asperities at a border between adjacent recording bits need to be reduced by decreasing the sizes of the magnetic grains. However, reducing the magnetic grains in size leads to a decrease in the volumes of the magnetic grains, and thereby drawbacks that thermal stability of magnetization in the magnetic grains decreases occur.
As a countermeasure against this problem, increasing magnetic anisotropy energy Ku of magnetic grains may be considered; however, the increase in Ku causes an increase in an anisotropic magnetic field (coercive force) of the magnetic recording medium. On the other hand, the upper limit of the recording magnetic field intensity of the thin film magnetic head is substantially determined by saturation magnetic flux density of a soft magnetic material configuring a magnetic core in the head. As a result, when the anisotropic magnetic field of the magnetic recording medium exceeds an acceptable value determined by the upper limit of the recording magnetic field intensity, it becomes impossible to record to the magnetic recording medium. Currently, as a method to solve such thermal stability problem, a so-called thermally-assisted magnetic recording method has been proposed in which, while a magnetic recording medium formed of a magnetic material with large Ku is used, under a state where the anisotropic magnetic field is reduced by heating the magnetic recording medium, a recording magnetic field is applied and the recording of information is performed.
In the thermally-assisted magnetic recording method, a method that uses a near-field light probe, which is a so-called plasmon-generator, that is a metal piece that generates near-field light from plasmon excited by laser light is generally known. As a magnetic recording head provided with such a plasmon-generator, proposed is a magnetic recording head provided with a magnetic pole, a waveguide, a plasmon-generator having a propagation edge opposing the waveguide (U.S. patent application Ser. No. 13/046,117).
In the thermally-assisted magnetic recording head, light propagating through the waveguide couples to the plasmon-generator in a surface plasmon mode so as to excite surface plasmon, and the surface plasmon propagates through the plasmon-generator (propagation edge), so that the near-field light is generated at the near-field light generating portion that is positioned in an air bearing surface side end part of the propagation edge. Furthermore, a magnetic recording medium is heated when the near-field light that is generated in the near-field light generating portion of the plasmon-generator is radiated to the magnetic recording medium, a magnetic field is applied under a state where an isotropic magnetic field of the magnetic recording medium is reduced, and thereby information is recorded.
In a thermally-assisted magnetic disk device provided with such a thermally-assisted magnetic recording head, characterizations of recording density, recording width, signal-to-noise (SN) ratio, bit error rate (BER), etc. are determined by receiving not only an effect from a magnetic field applied to the magnetic recording medium but also a thermal effect from radiated near-field light. Therefore, in order to evaluate the characterization of the thermally-assisted magnetic disk device, a parameter that is related to heat due to the radiated near-field light is needed to concern. Especially in recent years, steep magnetization reversal between adjacent recording bits of the magnetic recording medium and high recording density and high SN ratio are highly demanded, and a thermally-assisted magnetic recording device that can satisfy such demand has been developed. Through the development, a proposal of a method that highly precisely evaluates the characterization of the device upon a concern of a parameter related to heat has been demanded.